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Date:   Fri, 23 Jul 2021 08:03:42 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Mel Gorman <mgorman@...e.de>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH] mm,do_huge_pmd_numa_page: remove unnecessary TLB
 flushing code

Christian Borntraeger <borntraeger@...ibm.com> writes:

> On 22.07.21 02:26, Huang, Ying wrote:
>> Sean Christopherson <seanjc@...gle.com> writes:
>>>>
>>>> Thanks, I think you are correct. By looking into commit 7066f0f933a1
>>>> ("mm: thp: fix mmu_notifier in migrate_misplaced_transhuge_page()"),
>>>> the tlb flush and mmu notifier invalidate were needed since the old
>>>> numa fault implementation didn't change PTE to migration entry so it
>>>> may cause data corruption due to the writes from GPU secondary MMU.
>>>>
>>>> The refactor does use the generic migration code which converts PTE to
>>>> migration entry before copying data to the new page.
>>>
>>> That's my understanding as well, based on this blurb from commit 7066f0f933a1.
>>>
>>>      The standard PAGE_SIZEd migrate_misplaced_page is less accelerated and
>>>      uses the generic migrate_pages which transitions the pte from
>>>      numa/protnone to a migration entry in try_to_unmap_one() and flushes TLBs
>>>      and all mmu notifiers there before copying the page.
>>>
>>> That analysis/justification for removing the invalidate_range() call should be
>>> captured in the changelog.  Confirmation from Andrea would be a nice bonus.
>> When we flush CPU TLB for a page that may be shared with device/VM
>> TLB,
>> we will call MMU notifiers for the page to flush the device/VM TLB.
>> Right?  So when we replaced CPU TLB flushing in do_huge_pmd_numa_page()
>> with that in try_to_migrate_one(), we will replace the MMU notifiers
>> calling too.  Do you agree?
>
> Can someone write an updated commit messages that contains this information?

Hi, Andrew,

Can you help to add the following text to the end of the original patch
description?

"
The mmu_notifier_invalidate_range() in do_huge_pmd_numa_page() is
deleted too.  Because migrate_pages() takes care of that too when CPU
TLB is flushed.
"

Or, if you prefer the complete patch, it's as below.

Best Regards,
Huang, Ying

------------------------------------8<---------------------------------------------
>From a7ce0c58dcc0d2f0d87b43b4e93a6623d78c9c25 Mon Sep 17 00:00:00 2001
From: Huang Ying <ying.huang@...el.com>
Date: Tue, 13 Jul 2021 13:41:37 +0800
Subject: [PATCH -V2] mm,do_huge_pmd_numa_page: remove unnecessary TLB flushing
 code

Before the commit c5b5a3dd2c1f ("mm: thp: refactor NUMA fault
handling"), the TLB flushing is done in do_huge_pmd_numa_page() itself
via flush_tlb_range().

But after commit c5b5a3dd2c1f ("mm: thp: refactor NUMA fault
handling"), the TLB flushing is done in migrate_pages() as in the
following code path anyway.

do_huge_pmd_numa_page
  migrate_misplaced_page
    migrate_pages

So now, the TLB flushing code in do_huge_pmd_numa_page() becomes
unnecessary.  So the code is deleted in this patch to simplify the
code.  This is only code cleanup, there's no visible performance
difference.

The mmu_notifier_invalidate_range() in do_huge_pmd_numa_page() is
deleted too.  Because migrate_pages() takes care of that too when CPU
TLB is flushed.

Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
Reviewed-by: Yang Shi <shy828301@...il.com>
Reviewed-by: Zi Yan <ziy@...dia.com>
Cc: Dan Carpenter <dan.carpenter@...cle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Christian Borntraeger <borntraeger@...ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc: Heiko Carstens <hca@...ux.ibm.com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Vasily Gorbik <gor@...ux.ibm.com>
---
 mm/huge_memory.c | 26 --------------------------
 1 file changed, 26 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index afff3ac87067..9f21e44c9030 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1440,32 +1440,6 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
 		goto out;
 	}
 
-	/*
-	 * Since we took the NUMA fault, we must have observed the !accessible
-	 * bit. Make sure all other CPUs agree with that, to avoid them
-	 * modifying the page we're about to migrate.
-	 *
-	 * Must be done under PTL such that we'll observe the relevant
-	 * inc_tlb_flush_pending().
-	 *
-	 * We are not sure a pending tlb flush here is for a huge page
-	 * mapping or not. Hence use the tlb range variant
-	 */
-	if (mm_tlb_flush_pending(vma->vm_mm)) {
-		flush_tlb_range(vma, haddr, haddr + HPAGE_PMD_SIZE);
-		/*
-		 * change_huge_pmd() released the pmd lock before
-		 * invalidating the secondary MMUs sharing the primary
-		 * MMU pagetables (with ->invalidate_range()). The
-		 * mmu_notifier_invalidate_range_end() (which
-		 * internally calls ->invalidate_range()) in
-		 * change_pmd_range() will run after us, so we can't
-		 * rely on it here and we need an explicit invalidate.
-		 */
-		mmu_notifier_invalidate_range(vma->vm_mm, haddr,
-					      haddr + HPAGE_PMD_SIZE);
-	}
-
 	pmd = pmd_modify(oldpmd, vma->vm_page_prot);
 	page = vm_normal_page_pmd(vma, haddr, pmd);
 	if (!page)
-- 
2.30.2

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